Crystal growth apparatus



Nov. 10, 1964 o. IMBER 3,

CRYSTAL GROWTH APPARATUS Filed July 26. 1960 INVENTOR i OSCAR IMBERATTORNEYj United States Patent 3,156,533 CRYSTAL GROWTH APPARATUS OscarImber, 12214 Kendall Court, Silver Spring, Md. Filed July 26, 1960, Ser.No. 45,508 3 Claims. (Cl. 23-273) (Granted under Title 35, US. Code(1952), see. 266) The invention described herein may be manufactured andused by or for the Government of the United States of America forgovernmental purposes without the payment of any royal-ties thereon ortherefor.

This application is a continuation-impart of application Serial No.693,496, filed October 30, 1957, and now abandoned, for Crystal GrowthApparatus.

. The present invention relates generally to a crystal growing apparatusand more particularly to an improved apparatus for flameless fusiongrowth of single crystals wherein the molten material is maintained inminimum amounts during the crystfl growth.

The flameless fusion technique has been found most useful in growingcrystals, which, because of their chemical properties, cannot meet therigorous requirements of flame fusion and exposure to reducingatmospheres, hence crystal growths are produced by melting finelydivided particles and successively adding molten droplets to a growingcrystal boule in an inert atmosphere or in a vacuum. Although crystalssuch as germanium and silicon have been produced by this method it hasbeen the general practise to provide a melt in a relatively largequantity, for prolonged periods, and often at temperatures which affectcrucible materials. The introduction of undesirable impurities from thewalls of a crucible and the drastic conditions of the molten state oftenaffect the stability of the melt and lead to unpredictable results.Previous devices have been designed which maintain the crystallizablematerial as a powder and which meter said powder through a heated zoneto the growing crystal, but the solid and semi-solid particles whichreach the region of crystal growth require additional heating toestablish a molten state on the crystal surface.

The general purpose of this invention is to provide an improved crystalgrowing apparatus to overcome the disadvantages described above byminimizing both the quantity and duration of the molten material withinthe crucible.

It is also an object of the invention to provide an improved crystalgrowing apparatus which supplies a molten nutrient that is free of solidparticles or impurities.

It is another object of the invention to provide an improvement in acrystal growing apparatus wherein the crucible is readilyinterchangeable to meet various conditions of crystal growth and toincrease the versatility of the apparatus for growing various types ofcrystals.

A further object is to provide an improved apparatus for the fiamelessfusion technique which is easier to operate resulting in improved yieldsof usable products.

Other objects and advantages of the invention will become more 'fullyapparent in the following detailed description with reference to theaccompanying drawing, wherein: I g

The figure is a vertical cross-section view of the apparatusillustrative of the invention.

Broadly stated the invention refers to an improvement in an apparatusfor growing crystals which provides a conical crucible supported withina cylindrical chamber, and a powder dispensing means, or hoppersuspended in the upper part of the chamber directly above theconicallyshaped crucible. A discharge opening in the apex of theconically-shaped crucible communicates with the lower part of thechamber where a seed crystal is supported directly beneath the opening.The conical member is read- Patented Nov..10, 1964 ily interchangeablethus providing crucible structures of various degrees of inclination,composition and discharge opening. In operating the device, powderspills from the hopper onto the hot conical crucible where the powdermelts and collects in a molten pool over the discharge opening of thecone. opening to the crystal grown below.

An important feature of the present device resides in the cruciblestructure; conical structures in the past have been merely funnelingdevices. In the present structure the powder which falls into theconical crucible is melted and retained therein in a relatively smallamount. The crucible opening at the apex has a diameter size whichprevents the flow of melt until a sufiicient amount has accumulatedtherein to allow droplets to form and fall through said opening. It hasbeen found that a discharge opening of from about 0.5 mm. to about 1.1mm. is sufficient to prevent initial flow of melt in the absence ofsufiicient hydrostatic pressure.

Referring now to the drawing, a cylindrical quartz jacket 11 is mountedon a base plate 12 which provides a closure means at the base of thecylinder and supporting means for the entire apparatus. A detachablecover plate 13 provides sealing means at the top of the cylinder andalso a support for the powder dispensing means or hopper 14 which isadapted to receive a powder 15 through cover plate 16. A tapperarrangement in the hopper is designed to spill powder into a conicallyshaped crucible 17. The tapper arrangement includes a movable plate 18positioned across a relatively wide mouth open ing at the base of thehopper and connected by a rod 21 to an anvil plate 22. A spring typebellows positioned between the anvil plate 22 and cover plate 16 urgesthe movable plate 18 tightly against the mouth of the hopper by means ofrod 21. The powder 15 spills from the mouth around the outer edge of themovable plate 18 whenever the anvil plate 22 is tapped or vibrated whilebellows 23 acts to stop the downward movement of the tapper by restoringplate 18 to its original position after each tap. The conical crucible17 is positioned within the jacket directly below the hopper and restsupon an annular carbon ring 24. The ring has a center opening 29 and aslanting surface 26 on which the conical crucible rests and forms aclose alignment therewith while the apex portion of the crucibledescends into the central opening 29. The lower portion 28 of saidannular ring is flared out radially forming a centrally disposed domespace 39 into which the central opening 29 emerges. A support rod 27extends vertically through base plate 12 to provide supporting means forseed crystals 38 on which is grown the 1, crystal boule 3 1. The supportrod is positioned directly below and in line with opening 25 and isadjustably mounted tobe raised and lowered by adjustable means 33 of anywell known type.- An inlet gas tube 36 at the upper end of the cylinderand an outlet gas tube 37 at the lower end thereof allow for an inertgas to circulate within the cylinder space. Sealing means 35 positionedaround the movable support rod, below the base plate prevents escape ofthe enclosed gas.

An induc-tion'coil 34, surrounding the cylindrical jacket at theapproximate location of the annular carbon ring within the cylinderprovides for induction heating of the carbon ring.

In growing crystals of germanium, silicon or of alkali metal halidesthe, conditions of crystal growth may be varied depending upon thecomposition of the melt, the

' inert dry atmosphere which is employed, the particular opening at theapex of the cone and the material of which the cone is made. The conicalmember therefore, which is provided in the present apparatus, is madeinterchangeable and the crucible composition, opening at the apex andconical angle may be varied to meet the requirements Drops form and fallthrough the of various conditions for crystal growth. For example,silicon crystal growth-requires acme made of silica composition having adischarge opening at the apex of the cone in the range of about 0.7 mm.to about 1.1 mm. in diameter. In utilizing the same apparatus forgermanium crystal growth, it has been found necessary to employ a conestructure made of carbon; the opening at the apex is preferably in therange of about 0.5 mm. to about 0.8 mm. in diameter. Again when theapparatus is converted for alkali metal halides, the crystals are grownby providing a platinum crucible; the melt of the halides isconsiderably more fluid, thus, the conical crucibles have dischargeopenings of about 0.5 mm. in diameter.

In growing crystals by means of the present apparatus, the powdermaterial must have a high degree of purity, although the particle sizethereof is not very critical. Most satisfactory results are obtainedfrom particles some where in the order of about 50 to 80 standard mesh.Variations in the particle size will not interrupt crystal growth, sincethe particles melt in the cone region and only droplets of melt ofuniform size will fall through opening 25 in uniform droplet size. Theopening, therefore, in the apex of the conical crucible is of a certaindimension, such that the molten substance will be retained within thecone, the surface tension of the liquid preventing any initial flow.When the melt has built up sufficiently above the discharge opening, thehydrostatic pressure of the melt increases to the point where it exceedsthe surface tension of the melt, a droplet forms and falls through theopening to the crystal surface below. Discharge openings in the range offrom about 0.5 millimeter to about 1.1 millimeters have been found tooperate essentially in this manner.

In addition to controlling and minimizing the melt present in the cone,the conical hearth is especially useful for retaining any impuritiesthat may be present in the melt. Impurities appear in the form of slagwhich float in the melt and are thus retained in the conical vessel nearthe surface of the melt.

In operating the apparatus, a seed crystal 3-8 is positioned initiallyon the tip of support rod 27, and powder material suitable for crystalgrowth is placed in the hopper. The induction coil 34 is connected to anelectric source and the annular ring 24 becomes inductively heated, andthe conical crucible i7 is, in turn, raised in temperature sufficient tomelt the powder. Any suitable mechanical means may be rigged to vibrateor tap the anvil plate 22 whereby a powder, such as germanium orsilicon, falls in small increments onto the heated crucible 17 when theanvil plate of the tapper arrangement is vibrated or tapped. The mouthof the hopper is broad enough so that the powder that spills frommovable plate 18 falls near the base of the conical surface, that is,away from the apex region of the cone. The powder, after it hascontacted the heated surface, melts to form droplets that roll towardthe apex to form a molten mass 30 above opening 25. The molten massforms droplets at the opening when the melt has sufficient weight toovercome its surface tension. Droplets will form and fall throughopening 25 to the molten surface 32 on crystal boule 31.

The support rod on which the crystal is grown is moved vertically withinthe dome space near the heat zone, and it is initially adjusted to bringthe molten portion of boule 32 to the desired heat zone in the domespace. Once the optimum temperature for crystal growth is obtained, thesupport rod is lowered slowly as the crystal boule is growing to keepthe molten part 32 at approximately the same level within the dome. Theheat surrounding the crystallizing surface remains constant during theentire operation.

Inlet 36 and outlet 37 at the top and bottom of the 1 cylinder,respectively, provide means through which helium gas or some other inertgas is circulated about the heat zone and the cooling portion of thecrystal. The presence of an inert gas prevents oxidation interferenceduring the crystal growth and minimizes contamination of the growingcrystal. Although the apparatus is shown operating with inert gas, it isof course understood that the apparatus can be readily converted tovacuum operation.

Obviously many modifications and variations of the present invention areposisble in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practised otherwise than as specifically described.

What is claimed:

1. A crystal growth apparatus comprising:

(a) a chamber having a vertical axis,

(1)) a conical container having an open upper base, said base being inopen communication with said chamber, an inner lateral surface and asuitable opening at the apex thereof,

(0) said opening having a dimension approximately in the range of about0.5 mm. to about 1.1 mm. to permit gravity flow under a hydrostatichead,

((2) means arranged above said container for periodically dischargingpowder on said surface,

(e) heating means provided at the periphery of said container, formelting the powder contacting said surface,

(f) a support rod below said opening adapted to support a crystal growthelement thereon,

(g) said conical container, said means for discharging powder and saidsupport rod being supported in said chamber and arranged insubstantially coaxial relationship to said axis,

(11) means coupled with said chamber for moving said rod downward awayfrom said opening, and

(i) means connected to said chamber for circulating an inert atmospheretherethrough.

2. A crystal growth apparatus comprising:

(a) a chamber having a vertical axis,

(1)) a conical crucible having an open upper base, said base being inopen communication with said chamber, an inner lateral surface and asuitable opening at the apex thereof,

(0) said opening having a dimension approximately in the range of about0.5 mm. to about 1.1 mm. to permit gravity flow under a hydrostatichead,

(d) a hopper arranged above said crucible and adapted to dischargepowder periodically on said surface,

(2) a carbon annulus contacting said crucible over a considerable areathereof and adapted to be inductively heated to convert the powdercontacting said surface into a melt,

(f) a support rod below said opening adapted to support a crystal growthelement thereon,

(g) said crucible, said hopper and said support rod being supported insaid chamber and arranged in substantially coaxial relationship to saidaxis,

(h) means coupled with said chamber for moving said rod downward awayfrom said opening, and

(1') means connected to said chamber for circulating an inert atmospheretherethrough.

3. A crystal growth apparatus comprising:

(a) a chamber having a vertical axis,

(b) a conical crucible having an open upper base, said base being inopen communication with said chamber, an inner lateral surface and asuitable opening at the apex thereof,

(0) said opening having a dimension approximately in the range of about0.5 mm. to about 1.1 mm. to permit gravity flow under a hydrostatichead,

(d) a hopperarranged above said-crucible, said hopper having awide-mouth opening at the base thereof, a movable cover plate over saidwide-mouth opening and means for moving said plate periodically todischarge powder circumferentially from said widemouth opening to saidsurface,

(e) a support rod below said opening adapted to support a crystal growthelement thereon,

(f) an inductively heated annulus having an upper surface in contactwith said crucible over a considerable area thereof and a lowerspherical cavity for heating the area of said rod,

(g) said crucible, said hopper and said support rod being supported insaid chamber and arranged in substantially coaxial relationship to saidaxis,

(h) means coupled with said chamber for moving said 10 rod downward awayfrom said opening, and

6 (i) means connected to said chamber for circulating an inertatmosphere therethrough.

References Cited in the file of this patent UNITED STATES PATENTS1,597,293 Ruif Aug. 24, 1926 2,398,952 Nachod Apr. 23, 1946 2,935,386Selker May 3, 1960 FOREIGN PATENTS 774,270 Great Britain May 8, 1957901,413 Germany Mar. 15, 1954

1. A CRYSTAL GROWTH APPARATUS COMPRISING: (A) A CHAMBER HAVING AVERTICAL AXIS, (B) A CONICAL CONTAINER HAVING AN OPEN UPPER BASE, SAIDBASE BEING IN OPEN COMMUNICATION WITH SAID CHAMBER, AN INNER LATERALSURFACE AND A SUITABLE OPENING AT THE APEX THEREOF, (C) SAID OPENINGHAVING A DIMENSION APPROXIMATELY IN THE RANGE OF ABOUT 0.5 MM. TO ABOUT1.1 MM. TO PERMIT GRAVITY FLOW UNDER A HYDROSTATIC HEAD, (D) MEANSARRANGED ABOVE SAID CONTAINER FOR PERIODICALLY DISCHARGING ON SAIDSURFACE, (E) HEATING MEANS PROVIDED AT THE PERIPHERY OF SAID CONTAINER,FOR MELTING THE POWDER CONTACTING SAID SURFACE, (F) A SUPPORT ROD BELOWSAID OPENING ADAPTED TO SUPPORT A CRYSTAL GROWTH ELEMENT THEREON, (G)SAID CONICAL CONTAINER, SAID MEANS FOR DISCHARGING POWDER AND SAIDSUPPORT ROD BEING SUPPORTED IN SAID CHAMBER AND ARRANGED INSUBSTANTIALLY COAXIAL RELATIONSHIP TO SAID AXIS, (H) MEANS COUPLED WITHSAID CHAMBER FOR MOVING SAID ROD DOWNWARDLY AWAY FROM SAID OPENING, AND(I) MEANS CONNECTED TO SAID CHAMBER FOR CIRCULATING AN INERT ATMOSPHERETHERETHROUGH.